1. Field of the Invention
This invention relates to nuclear equipment and more particularly to methods and apparatus for the non-destructive inspection, manipulation, disassembly and assembly of reactor fuel elements, and the like.
2. Description of the Prior Art
To generate power in a nuclear reactor, an adequate quantity of nuclear fuel or fissionable material must be concentrated in the reactor core. This concentration produces heat through a sustained sequence of reactions between neutrons and fissionable nuclei within the core. These fission processes, moreover, leave a residue of nuclear debris within the reactor core that is intensely radioactive.
In order to assemble a concentration of this nuclear fuel in a sufficient quantity to propagate the fission process, the fissionable material usually is pressed into pellets, which are loaded into and sealed within hollow metal rods or "pins." The individual pins ordinarily are assembled into a group that constitutes a "fuel element." An array of these fuel elements, when assembled in a side-by-side relation, generally constitute the active portion of the reactor core. Because of the high pressures, temperatures, and other hostile environmental conditions that often are encountered within a reactor core it is occasionally necessary to inspect the individual fuel pins to insure that they have not lost their structural integrity.
As a part of the fuel element construction, the individual pins are held in place through two end fixtures that each engage respective pin extremities. Cellular grids also are positioned at intervals along the tubular surfaces between the end fixtures in planes that are generally transverse to the longitudinal pin axes. Typically, these grids are rectangular arrays of interlocking metal plates. The arrays of interlocking plates thus form more or less cube-like cells that receive the individual fuel pins. Protrusions jut from the surfaces of the plates that form the cells. These protrusions engage the adjacent surfaces of the fuel pins and hold the respective pins rigidly in place. Because of the radioactivity that is induced in these fuel elements as a result of nuclear processes, it is necessary to dismantle used and partially used fuel elements under at least ten feet of water. The water, in this instance, provides radiation protection. Clearly, disassembly and re-assembly operations in these circumstances must be conducted in a tedious, painstaking manner with remotely handled tools and specially trained crews. This situation is further aggravated by the conflicting requirements for a structurally strong fuel element that is capable of withstanding about three years exposure to the extreme physical conditions within a reactor core, and nevertheless be readily dismantled through remote manipulations.
Because the need for structural integrity has been paramount, fuel element disassembly heretofore has involved the destruction of many of the components in order to remove, identify and separate those few fuel pins that might require replacement. This is an expensive and wasteful process, especially in view of the high cost of these components. Thus, there is a need for a safe, efficient, and essentially non-destructive technique for dismantling fuel elements.